Some former force rebalance tunneling sensors used a single capacitor with a square law volts-to-force relationship. This yielded an output voltage proportional to the square root of the quantity to be measured. Alternative former force rebalance tunneling sensors used piezoelectric layers to perform the rebalance function.
Sensors providing a non-linear output are generally undesirable because they lead to harmonic distortion of the quantity being measured. Sensitivity also varies with the magnitude of input signals. Furthermore, the dynamic range over which a sensor yields a faithful representation of an input signal is reduced.
Piezoelectric rebalancing is generally inferior due to hysteresis, poor temperature and time stability, and small available displacements. The additional complexity of fabricating piezoelectric layers on a micromachined device is also undesirable.
On the other hand, linear force rebalancing increases dynamic range and reduces non-linearity, harmonic distortion, and intermodulation distortion. For many applications, such as phased arrays, linear operation is absolutely essential.
NASA's Jet Propulsion Laboratory (JPL) has designed a state of the art tunneling accelerometer device primarily for use in phased arrays (see "Tunnel-Effect Displacement Sensor", NASA Tech Briefs, Vol. 13, No. 9, Sep. 1989), but this device has several minor drawbacks that may act as barriers to practical use. For instance, the JPL device requires a high bias voltage. Specifically, the JPL device currently requires a 200 volt bias voltage to close the gap between the tunnel-effect tip. This large voltage is necessary because of a large capacitor gap (hundreds of microns) in the rebalance capacitor. This is an uncommonly high voltage for use in towed arrays, inasmuch as high voltages create corrosion and safety hazards in handling and testing. Furthermore, the circuitry required to generate such a high voltage can generate noise for the rest of the array.
Another drawback of the JPL device is that it employs non-linear force rebalance. A single capacitor is used for force rebalance in the JPL device, and the force across this single capacitor is proportional to the square of the applied voltage. This puts a non-linearity in the feedback loop wherein the output voltage is proportional to the square root of the incident acceleration. This, in turn, creates harmonic distortion, intermodulation, and phase non-linearity, which leads to reduced sensitivity and dynamic range. For array applications, linearity, uniform phase, and low distortion are essential to the combining of the numerous transducers which make up the array.
Still another drawback of the JPL device is its size, which is on the order of 8 mm. This is fairly large for a micromachined sensor. For many applications, such as thin line towed arrays, this is simply too large.
Accordingly, it would be desirable to overcome the disadvantages of former force rebalance tunneling sensors and thereby provide a tunneling sensor having a pair of force rebalance capacitors that are used in a push-pull relationship so as to provide a rebalance force that is a linear function of applied rebalance voltages, which leads to an output torque voltage that is linearly related to input acceleration.